Why does the frequency of power generation change when there is a change of load on the power grid?

Question

Back ground : Prime Minister Narendra Modi’s appeal to citizens to switch off lights across India on Sunday to mark the coronavirus fight is generating a lot more work and some tough challenges for India’s power sector workers.

Now it causes a lot of debate about power grid failure and complete black out in India. News here.

From the above link a small part

After that, machines shall be withdrawn from the grid through under frequency relays graded between 49.90 to 49.70 Hz amongst the units at each of the pumped hydro stations.

Now the question is why does the frequency of power generation change when there is a change of load on the power grid? Shouldn't the generators be running independent of the load?

Answer 1

why does the frequency of power generation change when there is a change of load on the power grid?

It's a classic control system problem and nothing to do with any background stated. With finite gain error amplifiers, a finite error is required to drive the system into near-lock. The error amplitude is the amount of frequency deviation from 50 Hz being currently produced - that error drives the generators to speed up or slow down. Under transient conditions this is exaggerated because there is lag in the system and things that weigh hundreds of tonnes take time to catch up to speed.

Without a control system, a generator would make a very poor generator in that as you add load it will slow down and, to speed it back up you need to apply more mechanical effort (more coal, more gas, more steam or more water through the hydro turbines).

Keeping the voltage constant (at source) is easier because you can use field winding control to increase or decrease the generated voltage but, frequency depends on the spinning rate of the generator.

Answer 2

Synchronous generators have certain consumption dependent phase lead when they pump energy to the grid. Magnetization control affects only reactive power.

Burning fuel in motors, letting water to flow etc provide the torque which makes possible the generators lean forwards the right phase lead. If the consumption drops suddenly the torque is enough to speed up the generator, the frequency rises.

The control systems in different power plants have different ability to control the situation. There's no grid wide central command system which can reduce the torque in every generator synchronously with the consumption changes, central commands affect slower. Local control systems with their own delays try to perform the central command orders which are more like "produce this amount of power and the planned frequency and voltage are these"